1. Field of the Invention
The invention relates to a fuel injection control apparatus and a fuel injection control method for an internal combustion engine, which control the condition of combustion between stratified-charge combustion and homogeneous-charge combustion by changing a mode in which fuel is injected.
2. Description of the Related Art
Japanese Patent Application Publication No. 2005-220887 (JP-A-2005-220887) describes a fuel injection control apparatus for an internal combustion engine, which supplies fuel to a combustion chamber by selectively using an injector for cylinder injection and an injector for intake port injection, according to the operating condition of the internal combustion engine. When the operating condition of the internal combustion engine is changed from an operating condition where one of the injectors injects the fuel to an operating condition where the other injector injects the fuel, the fuel injection control apparatus gradually decreases the proportion of the amount of fuel to be injected from the one injector, and gradually increases the proportion of the amount of fuel to be injected from the other injector during a predetermined transitional period to suppress fluctuations of torque. In addition, Japanese Patent Application Publication No. 2000-8916 (JP-A-2000-8916) describes a technology related to the invention.
An example of the fuel injection control apparatus that includes two types of injectors, that is, the injector for cylinder injection and the injector for intake port injection has been put into practical use. The fuel injection control apparatus uses the fuel injection from the injector for intake port injection and the fuel injection from the injector for cylinder injection in combination for one combustion stroke, to achieve the optimum condition of combustion. In the fuel injection control apparatus, the condition of combustion is changed according to an injection distribution ratio, that is, a ratio between the amounts of fuel to be injected from the injectors. The amount of pollutants discharged from the internal combustion engine is changed according to the change in the condition of combustion. Accordingly, an injection distribution ratio that is appropriate for an operating condition where the fuel injection from the injector for intake port injection and the fuel injection from the injector for cylinder injection are used in combination (for example, a first idling condition after the start of the engine) is determined in advance, for example, through an adaptability test, and the determined value is set as a target injection distribution ratio. The fuel injection operations of the injectors are controlled to achieve the target injection distribution ratio.
When an engine condition at the time of start of the engine differs from the engine condition based on which the target injection distribution ratio is determined, and the fuel injection control is executed according to the target injection distribution ratio, the optimum condition of combustion may not necessarily be achieved. For example, it may be not appropriate to control the injection distribution ratio to the target injection distribution ratio, due to the influence of a lean limit that is one of factors that impose restrictions on the combustion control. The lean limit is the limit value of an air-fuel ratio, above which combustion is not performed. That is, if the air-fuel ratio is on the lean side of the lean limit (the air-fuel ratio is above the lean limit, i.e., the amount of air is excessively large with respect to the amount of fuel), combustion is not performed. The lean limit moves toward a rich side (i.e., the lean limit decreases), as an engine temperature decreases. Therefore, when an engine temperature at the time of the start of the engine is below the engine temperature based on which the target injection distribution ratio is set, and the injection distribution ratio is controlled to the target injection distribution ratio, the air-fuel ratio exceeds the lean limit, and combustion may not be performed. In this case, it is effective to control the injection distribution ratio to promote the stratification of the air-fuel mixture and to ensure that the air-fuel mixture at the air fuel ratio equal to or below the lean limit is present around an ignition plug, by changing the injection distribution ratio so that the amount of fuel to be injected from the cylinder injection is increased.
However, the injection distribution ratio only temporarily needs to be changed from the target injection distribution ratio. Thus, it is preferable to return the injection distribution ratio to the target injection distribution ratio when the engine condition is changed to the predetermined engine condition based on which the target injection distribution ratio is set. In this case, when the injection distribution ratio is changed only in the predetermined transitional period as described in the publication No. 2005-220887, the injection distribution ratio is not necessarily changed in accordance with the change in the engine condition in the predetermined transitional period. As a result, for example, a misfire may occur, or the amount of pollutants contained in exhaust gas may be increased, depending on the operating condition of the engine.